1. Field of the Invention
The present invention relates to a hydraulic lift structure, and more particularly to a layout structure of an oil reservoir of a working fluid which is supplied to and discharged from a hydraulic cylinder case (a hydraulic cylinder) moving up and down a working machine, in a working vehicle structured such that the working machine is replaceable and detachable such as a back hoe, a rotary power tiller or the like.
2. Background Art
The working vehicle such as a tractor or the like is structured such that the working machine such as the rotary power tiller or the like is arranged in a rear side of a machine body, and a fertilizer applicator, a seeding machine or the like can be replaced and attached in addition to the rotary power tiller or the like, in accordance with a working content. For example, as the tractor in which the unit such as the rotary power tiller or the like is arranged in a rear side of the machine body, and the working vehicle in which the working machine is arranged in a front side and a rear side of the machine body, there is known a back hoe loader in which a front loader is arranged in the front side of the machine body and a back hoe is arranged in the rear side of the machine body, or the like.
In the working machine mentioned above, the rotary power tiller or the like is structured such that the working machine is lifted upward in such a manner that the working machine is not in contact with a ground level at a time of a normal traveling of the working vehicle, and is descended at a time of executing a work so as to be grounded on a farm field. Further, the structure is made such that its upper and lower positions are adjusted in correspondence to a state of the farm field surface and in such a manner as to achieve a target depth of plowing during the operation. In order to make it possible to move the working machine up and down, a hydraulic cylinder coupled to a lift arm and moving up and down the working machine by a hydraulic pressure is normally arranged in a rear portion of a transmission case, whereby a hydraulic lift structure is formed.
In the hydraulic lift structure of the working vehicle mentioned above, there has been generally proposed a structure in which a hydraulic cylinder or the like is provided inside a hydraulic cylinder case, the hydraulic cylinder case is arranged at a lower position of a driver seat in an upper side of a rear portion of a transmission case, and the hydraulic cylinder case is detachable with respect to the transmission case (refer to Patent Documents 1 and 2).
Further, as disclosed in Patent Document 3, a working fluid in an oil reservoir formed within the transmission case is pressure fed and supplied to a hydraulic cylinder, a hydraulic valve or the like arranged in the hydraulic cylinder case by a hydraulic pump or the like, and a return fluid from the hydraulic cylinder, the hydraulic valve or the like is again returned into the transmission case via the hydraulic cylinder case (refer to Patent Document 3).
Patent Document 1: Japanese Unexamined Patent Publication No. 8-104149
Patent Document 2: Japanese Unexamined Patent Publication No. 7-39201
Patent Document 3: Japanese Unexamined Utility Model Publication No. 5-31507
Of course, as disclosed in Patent Document 1 and Patent Document 2, it is possible to improve a maintenance characteristic of the working vehicle and make an assembling work or the like easy by setting the hydraulic cylinder case (the hydraulic cylinder) structured such as to be detachable with respect to the transmission case.
However, in the conventional hydraulic lift structure, as disclosed in Patent Document 3, the working fluid supplied to and discharged from the hydraulic cylinder is used in common with the working fluid supplied to and discharged from the transmission. Accordingly, a dust garbage (a contamination) generated from a mechanical gear mechanism such as a sub transmission, a PTO transmission, a differential gear or the like constituting the transmission mixes into the hydraulic cylinder case, and there is a case that the hydraulic cylinder gets out of order due to the dust garbage.
Further, in the conventional hydraulic lift structure, in order to prevent the contamination mentioned above, a line filter is arranged in an oil path for supplying and discharging the working fluid in addition to a suction filter. Accordingly, an oil path pressure is lowered by arranging the line filter, and a temperature of the working fluid is ascended so as to generate a reduction of a power transmission efficiency, a reduction of a fuel consumption or the like. Further, a manufacturing cost of the working vehicle (the hydraulic lift structure) becomes higher by arranging the line filter.
A problem to be solved is to propose a hydraulic lift structure which prevents a malfunction of a hydraulic cylinder or the like and improves a supplying and discharging efficiency of a working fluid.
The problem to be solved by the present invention is as mentioned above, and a description will be given next of a means for solving the problem.